A wireless communication unit usually forms part of a wireless communication system. The wireless communication unit communicates through a wireless communication network, which also forms part of the wireless communication system.
The wireless communication network normally comprises a network of base stations. Each base station enables communication within an area referred to as a cell-site. Each cell-site may comprise multiple sectors. There are usually three sectors in a cell-site, each served by a dedicated antenna.
In some countries, it is a legal requirement that wireless communication systems be able to provide accurate information about the location of wireless communication units. This information may serve, for example, to expedite the arrival of assistance to a user of a wireless communication unit who calls the emergency services, using the ‘911’ number in the United States.
Information about the location of a wireless communication unit can be derived in many ways. At any particular time, various forms of measurement information may be available from:
(i) The wireless communication unit;
(ii) The wireless communication network, and particularly from one or more base stations with which the wireless communication unit is communicating; or
(iii) Both of (i) and (ii).
This measurement information can be processed to provide an estimate of the location of the wireless mobile communication unit.
Considering the measurement information in more detail, this information may be available either:
(i) Directly. This means that the information is included in the measurement made. The measurement may be made either by the wireless communication unit, or by another part of the wireless communication system, such as the wireless network.
(ii) Indirectly. This means that the information is derived from the measurements made. An example would be an estimate of the distance between a wireless communication unit and the base station of a wireless communication system. Such an estimate might be calculated by multiplying the speed of propagation of the signal by a measured time difference between transmission and receipt of a signal.
So some or all of the following mobile measurement information may be available:
(i) The absolute distance(s) from the wireless communication unit to one or more network sectors.
(ii) Differential distances between the wireless communication unit and one or more pairs of network sectors.
(iii) Received signal powers recorded by the wireless communications unit from one or more network sectors.
(iv) Received signal-to-noise ratio measurements recorded by the wireless communication unit from one or more network sectors.
In addition, the following network information may be available:
(i) Antenna locations per sector. This information may be provided in latitude and longitude, or as ‘Easting’ and ‘Northing’ directions, or the equivalent.
(ii) Antenna properties. This information may include height above ground, azimuth, tilt, horizontal and vertical beam pattern, transmitted power levels for control and traffic channels.
A combination of the mobile measurement information and the network information is often used to provide geometric interpretations of the mobile device location.
So prior art geo-location methods have as their inputs various forms of directly- and indirectly derived measurement information. That measurement information may have come from the wireless communication unit, the wireless communication system, or both. Some wireless communication units may be able to provide further information that originates from direct communication links between the wireless communication unit and other wireless communication units. Such direct communication links do not pass through the wireless communication network.
FIG. 1 shows a simple example of a wireless communication system 100. Wireless communication unit 110 communicates with base station 120. Base station 120 is one of many base stations that together comprise the wireless communication system 100. Base station 120 is the nearest base station to wireless communication unit 110, and is located a distance ‘R’ from wireless communication unit 110.
FIG. 1 shows one problem with prior art geo-location methods. If the base station 120 has an omni-directional antenna, and no other measurement information is available, then any attempt to provide a single estimate of the location of the wireless communication unit 110 is very difficult. The information available from the wireless communication unit 110 and/or from base station 120 allows prior art geo-location methods to calculate only the absolute value of distance R. This distance is measured relative to the known location of base station 120, which might be at coordinates (xB, yB). However, all that is known is that the wireless communication unit 110 is located somewhere on a circle of radius R, centred on (xB, yB), which is the circle 130 in FIG. 1.
The single estimate of the location would have to be a point chosen at random on the circle 130. The error in this estimate could be up to 2R, because the wireless communication unit might in fact be located on circle 130 at a point diametrically opposite to the estimated location.
FIG. 2 considers another simple example of a wireless communication system 200. FIG. 2 also shows a wireless communication unit 210 and base station 220. Base station 220 has a directional antenna. It is statistically most likely that wireless communication unit 220 is being served by the main lobe 230 of the directional antenna. So a single estimate of the location of the wireless communication unit 210 can be made with greater certainty than was the case with the wireless communication system of FIG. 1. However, it is also possible, but less likely, that wireless communication unit 210 is being served by a ‘side-lobe’ or ‘back-lobe’ of the directional antenna. A back-lobe is shown as reference 240 in FIG. 2.
A prior art geo-location method would provide an estimate of the location of wireless communication unit 210 based on the distance R and the direction of the antenna's main lobe. This estimate is shown as point (x1, y1) on FIG. 2. However, the wireless communication unit 210 might provide the same measurement information, such as a received signal to noise ratio, if it were in fact being served by the back-lobe 240 of the antenna. In this case, it would be located at a distance less than R from base station 220, in the opposite direction. Such a location is shown as point (x2, y2) on FIG. 2, which is located 0.25R from base station 220. If wireless communication unit 210 were in fact located at point (x2, y2), then the estimated location (x1, y1) would be in error by a distance of 1.25R.
In general terms, prior art geo-location techniques usually deliver a single point in space as their estimate of the location of the wireless device. This point may, for example, be described by an x coordinate and a y coordinate, as in FIGS. 1 and 2. However, such an approach does not give the user information about the reliability of the estimate of the location. Reliability, in this example, means both ‘accuracy’ and ‘precision’.
‘Accuracy’ concerns whether the estimated location is the correct one, or not. In the example of FIG. 1, it was explained that the measurement of the location of mobile communication unit 110 might be inaccurate by as much as 2R.
‘Precision’ is the exactness of the measurement. Both FIGS. 1 and 2 assumed that the distance R could be determined very precisely, i.e. that an exact value for R could be derived.
Both accuracy and precision depend on the type and quality of data on which the measurement estimate is based.
Whereas FIGS. 1 and 2 assumed that distance R could be determined reliably, there is in fact an error range associated with the measurement of R itself. The measurement of R is in fact imprecise.
FIG. 3 illustrates the imprecision in the measurement of R. FIG. 3 corresponds generally to the arrangement of FIG. 1. A base station 320 located at point (XB, YB) might measure the distance to a communication unit 310 as R. Circle 330 shows the locus of all points at the distance R, which is the distance from base station 320 at which the mobile is most likely to be located. Due to the measurement collection process, however, the user 310 might actually be at a distance of between (R−e1) and (R+e2) from base station 320. So it is not possible even to say with certainty that the user 310 lies on circle 330.
An error function describes the probability that communication unit 310 is located at each particular distance from base station 320. The error function is usually complex. The details of the error function can be determined through a variety of means. One option is to place calls from a small number of locations, and compare the measurement data with the known locations.
Summing up the discussion of FIGS. 1-3, there is both inaccuracy and imprecision in the measurement of the location of a mobile communications unit. So some degree of error is likely in the measurement of the position of a mobile communications unit in most cellular communications systems.
These problems are greatest in the unusual case of:
(i) A base station with an omni-directional antenna, such as in FIG. 1; and
(ii) When received signal strength is the only measurement data on which an estimate of the position of the mobile communications unit can be made.
In the preceding discussion and the discussion that follows, the term ‘communicating’ includes a variety of forms of communication. These forms include, but are not limited to, speech or data communication sessions on traffic channels, and communication on the control channel. So, for example, the communication may not require the user of a mobile telephone to actually place or receive a call. The communication may involve, for example, only the intermittent receipt by a mobile telephone of data over the wireless communication system's control channel.
Prior art U.S. Pat. No. 5,293,642 (Lo) describes a variety of approaches to estimating the location of a mobile communication unit. The approaches involve calculating a probability density function for the location of the mobile communication unit. A mobile station may be in communication with two or more base stations. In this case, several probability density functions can be calculated, each describing the location of the mobile calculated from measurements received from one of the base stations. These probability density functions may be combined, to provide a joint probability density function. U.S. Pat. No. 5,293,642 (Lo) may employ any of the following data in calculating a location probability density function:
Radio attenuation from RSSI measurements at the mobile station;
Radio attenuation from RSSI measurements at the base stations;
Direction of signal arrival at the base station;
Radio propagation delay from mobile signal arriving at the base station; and
Mobile transmission timing alignment.
Prior art United States patent application US2008080429 (Hart) describes a process of minimum variance location estimation in wireless networks. A probability density function is calculated, and a ‘probability surface’ is derived from the probability density function. A mean location of a wireless node is calculated from the probability surface.